An uncooled type (thermal type) infrared detection device is the device that absorbs and converts infrared rays into heat by an infrared absorbing layer, and that converts the heat into an electrical signal by a thermoelectric conversion device. In the uncooled type infrared detection device, a surface microstructure or a bulk microstructure forming technology is used in order to thermally isolate the infrared absorbing layer and the thermoelectric conversion device from an external system. A cooled type (quantum type) infrared detection device requires an expensive and large cooler, while the uncooled type infrared detection device has an advantage of being small and inexpensive.
Sensitivity per one pixel of this uncooled type infrared detection device is as follows. Given that incident infrared power per unit area is Ilight, an infrared absorbance of a detection cell γ, an infrared absorption area per unit pixel AD, a thermal conductance from the detection cell to a semiconductor substrate Gth, and a thermoelectric conversion coefficient of a p-n junction dV/dT, an output signal Vsignal of a thermoelectric conversion section is expressed by Equation (1).Vsignal=(IlightADγ/Gth)(dV/dT)  (1)
In addition, when using a thermal capacity Cth and the thermal conductance Gth of the detection cell, a response characteristic of the infrared detection device is expressed by Equation (2).T=Cth/Gth  (2)
In Equation (2), T is a thermal time constant, and it denotes a time period until the output signal changes to be approximately 63% of a maximum value with respect to an incident signal, and provides an index of the response characteristic.
In order to improve the sensitivity of the infrared detection device, according to Equation (1), it is only necessary to increase the infrared absorption area AD, the infrared absorbance γ, and the thermoelectric conversion coefficient dV/dT of the p-n junction, and to reduce the thermal conductance Gth.
However, although the thermoelectric conversion coefficient dV/dT of the p-n junction is proportional to the number of diodes, increase in number of diodes leads to increase in driving power, causing increase of power consumption. In addition, since a dV/dT characteristic of a single diode is determined by bandgap energy of a semiconductor, significant increase thereof cannot be expected. When the thermal conductance Gth is reduced, sensitivity improves, but the thermal time constant drops, thus resulting in the reduced response characteristic. Increase of the infrared absorption area AD also increases a pixel pitch size by the increase, and the response characteristic is reduced due to increase in the thermal capacity. When the infrared absorbing layer is the single one, the infrared absorbance γ increases exponentially with respect to a thickness of the layer, and therefore, when a desired absorbance is tried to be obtained, thermal capacity increases accordingly, thus causing the reduced response characteristic. Namely, the sensitivity and the response characteristic are in a trade-off relationship. Hence, when priority is given to characteristic improvement of either one of the sensitivity and the response characteristic, the characteristic of the other one inevitably deteriorates.